This invention relates generally to porous articles, more specifically to porous articles formed from organic polymer and having functional groups at the surface thereof, and to methodology for preparing surface-functionalized porous articles using remote radical forming conditions.
Porous solid materials are widely utilized in the synthesis and purification of organic and biochemicals. For example, the synthesis of biochemicals such as oligonucleotides and polypeptides can be done using solid phase supports in order to greatly reduce the time and expense arising from the inevitable purifications needed between intermediate steps in a multi-step chemical synthesis. The practice of solid phase synthesis is also particularly amenable to automation, thus allowing significant cost-savings in terms of labor. These advantages have prompted a great deal of research activity directed to improvements in solid phase synthesis, and one area of keen interest is the development of superior solid supports.
Many different solid supports are currently used commercially for separation or solid phase synthesis, and many more have been described in various patents and publications. In general the efficiency of a solid phase synthesis and separation processes depends on the surface area of the solid phase material. Porous materials offer is the advantage of higher surface area per unit volume than the corresponding full density solids, permitting vastly improved synthesis and separation performance per unit volume of solid material.
One popular solid support is made from glass or silica, typically in the form of beads. Glass beads have several desirable properties. For example, they are inert to most (although not all) chemical reactions, and they are not easily crushed. Glass has many surface hydroxyl groups that can be used as xe2x80x9cchemical handlesxe2x80x9d to join molecular fragments to the glass. Glass is fundamentally inexpensive, and technology has developed that can make glass beads highly porous, so that the beads have a high surface area/volume ratio.
However, glass beads are also rather brittle, and thus not very stable to mechanical action. When placed into a container with a mechanical stirrer, small pieces of the beads typically chip away, and these small pieces (commonly termed xe2x80x9cfinesxe2x80x9d) may find their way into the filtration equipment that is used to separate the beads from the spent reaction solutions. These fines can clog the filtration equipment, and thus present a continuing maintenance problem.
Although there are a large number of organic polymers, in the form of beads, membranes or monoliths, that are not brittle, thereby providing researchers with a potential means for solving the xe2x80x9cfinesxe2x80x9d problem associated with glass beads, identifying an organic-based solid support with the correct balance of properties has proven to be a significant challenge. Typical methods to functionalize an organic polymer, i.e., provide the xe2x80x9cchemical handlesxe2x80x9d that are needed to allow the polymer to serve as a solid-phase synthesis support or chromatography media, are rather harsh and lead to undesirable degradation of the polymer.
There is thus a need in the art for solid-phase supports for synthesis and separations which overcome the disadvantages of the prior art materials. The present invention fulfills this need and provides further related advantages as described herein.
The invention provides a porous article having an exterior surface, a bulk matrix and pores extending from the exterior surface into the bulk matrix. The pores define an interstitial surface. The bulk matrix is formed, at least in part, of an organic polymer comprising carbon and hydrogen atoms. The exterior and interstitial surfaces are formed, at least in part, of the same organic polymer comprising carbon and hydrogen atoms, which has been modified so that some of the hydrogen atoms are replaced with functional groups including amino, hydroxyl, carbonyl, and carboxyl acid. The invention provides a mild method of introducing functionality to a porous article, so that the exterior surface does not display surface roughness, according to SEM analysis, due to ablation of surface carbon atoms or chain scission.
Another aspect of the invention provides a process for introducing functionality to the surface of a porous article. The process includes providing a precursor porous article having an exterior surface, an interstitial surface and a bulk matrix, where the interstitial surface and bulk matrix both are formed, in whole or part, of organic polymers. The porous article is exposed to a remote discharge formed by radical forming conditions acting on a source gas, where the source gas may be oxygen, ammonia or a mixture of nitrogen and hydrogen The porous article is exposed to the remote discharge under reaction conditions such that radicals from the discharge react with the organic polymers present at the exterior and interstitial surfaces of the precursor porous article. This reaction introduces direct covalent bonding of functionality to the surfaces, where the functionality may be amino, hydroxyl, carbonyl or carboxyl groups.
In one embodiment, the discharge used in the inventive process is a plasma discharge. In another embodiment, photons generated by the radical forming conditions, and particularly ultraviolet radiation, do not contact the precursor porous article. Subsequent to being exposed to the discharge, the surface-functionalized porous article having carbonyl and carboxylic acid groups (possibly in addition to hydroxyl groups) may be reacted with a reducing agent, so that hydroxyl groups are the predominant functional group bonded to the polymers which form the surface of the article. In addition, or alternatively, chemical agents that react with and cap (neutralize, eliminate) surface free radical sites and/or peroxides are contacted with the surface functionalized porous article. Such chemical agents include ammonia, dimethyl sulfide and other gases known in the art to cap/react with/neutralize free radical or peroxides on a polymer surface.
Another aspect of the invention provides a surface-functionalized porous article prepared by a process which includes providing a precursor porous article having an exterior surface, an interstitial surface and a bulk matrix. The surfaces and bulk matrix are formed, in whole or part, of organic polymers. The porous article is exposed to remote discharge formed by radical forming conditions acting on a source gas. The source gas may be oxygen, ammonia or a mixture of nitrogen and hydrogen. The porous article is treated under reaction conditions such that radicals from the discharge react with the surfaces of the precursor porous organic article to cause direct covalent bonding of functionality to the surfaces, where the functionality may be amino, hydroxyl, carbonyl or carboxyl groups.
In a preferred embodiment, analysis for surface functional groups introduced by the process described above shows that the same useful functional group concentration is present throughout the entire interstitial surface of the porous solid. The exterior pore surfaces preferably have the same or essentially the same morphology and functional group concentration as the internal pore surfaces. The internal and exterior pore surfaces of the porous articles of the invention show essentially the same morphology by SEM as the starting porous material for the process. Thus, the exterior pore surfaces and internal pore surfaces contain little or no degradation from: 1) UV/VUV photochemical degradation, 2) ion and electron impact processes, 3) chemical etching.
Polyolefin, and particularly polyethylene, is a preferred material from which the article is formed. Thus, the invention provides for porous articles having amino-substituted polyethylene forming the exterior and interstitial surfaces, and unsubstituted polyethylene underlying the surfaces. In another embodiment, the invention provides for porous articles having hydroxyl-substituted polyethylene forming the exterior and internal pores surfaces, and unsubstituted polyethylene underlying the surfaces. The surface amine and hydroxyl groups are distributed throughout the surfaces of the article, including the surface of the innermost pores.
These and other aspects of this invention will become apparent upon reference to the following detailed description and attached drawings.